Defining Terms

Continuous Culture-A large-scale closed system culture in which cells are grown in a fixed volume of nutrient culture medium under specific environmental conditions (e.g. nutrient type, temperature, pressure, aeration, etc.) up to a certain density in a tank or airlift fermentor, harvested and processed as a batch, especially before all nutrients are used up.

Dehydrogenase-(Science: enzyme) enzyme that oxidizes a substrate by transferring hydrogen to an acceptor that is either NAD/NADP or a flavin enzyme. An enzyme that is used to remove hydrogen from its substrate, which is used in the cytochrome (hydrogen carrier) system in respiration to produce a net gain of ATP.

Flux-(Science: radiobiology) The total amount of a quantity passing through a given surface per unit time. Typical quantities include (magnetic) field lines, particles, heat, energy, mass of fluid, etc. Common usage in plasma physics is for flux by itself to mean magnetic field flux, unless specified otherwise.

Metabolite-(Science: biochemistry) Any substance produced by metabolism or by a metabolic process. Any substance involved in metabolism (either as a product of metabolism or as necessary for metabolism). An end product as a result of metabolism.

Oligonucleotides-polymers made up of a few (2-20) nucleotides. In molecular genetics, they refer to a short sequence synthesised to match a region where a mutation is known to occur, and then used as a probe (oligonucleotide probes).

Permease-(Science: enzyme) general term for a membrane protein that increases the permeability of the plasma membrane to a particular molecule, by a process not requiring metabolic energy.

Physiological- Of, or pertaining to physiology or normal functioning of an organism.

Proline- (Science: amino acid) One of the 20 amino acids directly coded for in proteins. Structure differs from all the others, in that its side chain is bonded to the nitrogen of the amino group, as well as the carbon. This makes the amino group a secondary amine and so proline is described as an amino acid. Has strong influence on secondary structure of proteins and is much more abundant in collagens than in other proteins, occurring especially in the sequence glycine proline hydroxyproline. A proline rich region seems to characterise the binding site of SH3 domains. An amino acid that is found in many proteins (especially collagen).One of 20 amino acids commonly found as part of a protein.

Outline

Introduction

Ammonia is the preferred nitrogen source of yeast,Saccharomyces cerevisiae, as noted by rapid growth

The breakdown, or metabolism, of nitrogen is regulated at two levels: gene expression and enzyme activity

Previous studies have involved changing ammonia concentrations as well as flux

It is previously thought the flux of ammonia is more critical in nitrogen breakdown than ammonia concentration

This experiment will only change concentration as flux remains constant

Physiological parameters

A strain of yeast (SU32) was grown under the following conditions in continuous culture in a fermenter:

Different ammonia concentrations(mM): 29,44,61,66,78,90,96,114, and 118

Fixed glucose concentration (mM): 100

Such parameters allowed for an investigation of ammonia's influence on both gene expression and regulation

Fig 1A

X-axis shows ammonia concentration within the feed

Y-axis on the left shows residual ammonia concentration, or ammonia left in culture

Y-axis on the right shows biomass/ flux or the product of ammonia metabolism

Feed with 61mM ammonia or lower showed ammonia limitation

Feed with more than 61 mM ammonia resulted in glucose acting as the limiting factor

Fig 1B

X-axis shows which feed is used

Y-axis on the left shows the oxygen consumed and resulting carbon dioxide produced

Y-axis on the right shows the ratio of carbon dioxide produced over oxygen consumed

Feed with an ammonia concentration of 44mM or lower resulted in ammonia limitation